1. Field of the Invention
The invention relates to photosensitive, elastomeric, flexographic printing compositions based on asymmetric, multi-branched, block copolymers and the printing elements derived from such compositions.
2. Description of the Prior Art
Photosensitive printing plates used for flexographic printing are characterized by their ability to crosslink or cure upon exposure to actinic radiation. For the production of such printing plates, a template, such as a photographic negative or transparency, is put onto an unexposed plate containing a layer of photosensitive material. The plate is exposed to actinic radiation, such as a UV or black light. The actinic radiation enters the photosensitive material through the clear areas of the transparency and is blocked from entering the black or opaque areas. The exposed material crosslinks and becomes insoluble to solvents used during image development. The unexposed photopolymer areas under the opaque regions of the transparency remain soluble and are washed away with a suitable solvent leaving the desired image as a relief, an intaglio or a combination of both. Then the plate is dried. The printing plate can be further treated to remove surface tackiness and subsequently exposed to additional actinic radiation to ensure complete photopolymerization and final plate hardening. After these steps, the plate is mounted on a cylinder and used for printing.
Common defects of existing photopolymer printing plates are poor ink transfer and uneven impression. Other common defects include a lack of flexibility and resilience as well as poor durability. The present invention discloses compositions that exhibit significant improvement in these properties.
Photosensitive elements of the flexographic printing plates generally involve compositions containing an elastomeric or thermoplastic-elastomeric polymer; an addition polymerizable, nongaseous, ethylenically unsaturated, monomeric crosslinker and a photoinitiator activated by actinic radiation. Such photopolymer printing plates usually contain large amounts of the ethylenically unsaturated monomeric crosslinkers (10% and higher) that are undesirable due to the toxicity, high cost, and post processing extractibility of unpolymerized material.
One object of the invention is to produce a photosensitive composition that crosslinks with little or no ethylenically unsaturated crosslinking agents.
Another object of the invention is to produce a photosensitive composition and an element for flexographic printing that are soft, flexible, resilient and durable, and show no loss in dimensions or properties during use.
A third object of the invention is to produce a stable photosensitive composition and an element for flexographic printing. This composition requires no aging before use, yet can be stored for long periods of time without changing its photospeed and exposure latitude.